The present invention relates to powder coating compositions, particularly powder coating compositions with improved mar resistance properties.
Solid particulate coating formulations referred to in the industry as xe2x80x9cpowder coatingsxe2x80x9d can be applied over various substrates. A major benefit of powder coatings is that little, if any, volatile material is given off to the surrounding environment when powder coatings are cured. Due to increasing restrictions on volatile organic content (VOC), powder coatings are preferred for many applications.
One problem with conventional powder coatings is that they exhibit poor mar resistance properties. xe2x80x9cMar resistancexe2x80x9d refers to the ability of a coating composition to maintain its appearance when the coating comes in contact with an abrasive material.
To improve mar resistance, microparticulate materials such as silica, metal sulfides, and crosslinked styrene-butadiene are sometimes added to powder coating compositions. Because the addition of microparticulate materials adversely affects the gloss and the distinctness of image (DOI) of a coated surface, this is a less than ideal solution to the problem.
The present invention provides a curable powder coating composition which exhibits improved mar resistance properties.
The present invention is a curable powder coating composition comprising a polymer containing reactive functional groups, a curing agent having functional groups reactive with the functional groups of said polymer, the curing agent being present in an amount sufficient to cure said polymer, and less than 10 percent based on total resin solids of the powder coating composition of a tricarbamoyl triazine compound represented by the following chemical formula: C3N3(NRCOXY)3 where Y is an alkyl group or substituted alkyl group having 1 to 12 carbon atoms, X is NRxe2x80x2, O, S, PRxe2x80x2, and xe2x80x94Cxe2x80x94, and R and Rxe2x80x2 are hydrogen and alkyl or a substituted alkyl having 1 to 12 carbon atoms.
Various numerical ranges are disclosed in this patent application. Because these ranges are continuous, they include every value between the minimum and maximum values. Unless expressly indicated otherwise, the various numerical ranges specified in this application are approximations. It is implied that the minimum and maximum values within the stated ranges are preceded by the word xe2x80x9caboutxe2x80x9d. Therefore, slight variations above and below the stated ranges can be used to achieve substantially the same results.
The powder coating composition of the present invention comprises a polymer having reactive functional groups. The polymer having reactive functional groups can be chosen from a variety of materials, including but not limited to, acrylic polymers, polyurethane polymers, and polyester polymers. The reactive functional groups on the polymer are selected from carboxylic acid, epoxy, hydroxyl, amino, carbamate, and urea. The polymer is present in the powder coating composition in amounts ranging from about 10 to 90 percent by weight, based on the total weight of resin solids in the powder coating composition.
In an embodiment of the invention, the polymer having reactive functional groups is an acrylic polymer. Acrylic polymers containing the appropriate functional groups can be formed by reacting polymerizable alpha, beta-ethylenically unsaturated monomers containing the functional groups mentioned above with one or more other polymerizable, unsaturated monomers.
Suitable carboxylic acid group-containing monomers include acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, maleic acid, citraconic acid, and monoalkylesters of unsaturated dicarboxylic acids. Acrylic acid and methacrylic acid are the preferred carboxylic acids.
Suitable epoxy group-containing monomers include glycidyl acrylate and glycidyl methacrylate.
Suitable amino group-containing monomers include aminoethyl methacrylate and aminopropyl methacrylic.
Pendant carbamate functional groups can be incorporated into the acrylic polymer by copolymerizing the acrylic monomers with a carbamate functional vinyl monomer. Examples of suitable carbamate functional monomers include: (a) carbamate functional alkyl esters of methacrylic acid; (b) the reaction product of hydroxyethyl methacrylate, isophorone diisocyanate, and hydroxypropyl carbamate; (c) the reaction product of hydroxypropyl methacrylate, isophorone diisocyanate, and methanol; and (d) the reaction product of isocyanic acid with a hydroxyl functional acrylic or methacrylic monomer like hydroxyethyl acrylate.
Pendant urea groups can be incorporated into the acrylic polymer by copolymerizing the acrylic monomers with urea functional vinyl monomers. Examples of urea functional monomers include: (a) urea functional alkyl esters of acrylic acid or methacrylic acid and (b) the reaction product of hydroxyethyl methacrylate, isophorone diisocyanate, and hydroxyethyl ethylene urea.
The acrylic polymers typically have number average molecular weights of about 1,000 to 10,000 or 1,000 to 5,500 based on gel permeation chromatography using a polystyrene standard. The acrylic polymers have equivalent weights (based on the functional groups mentioned above) of about 200 to 400 or 250 to 355 gram/equivalent. The glass transition temperature (T(g)) of the polymer is typically about 30xc2x0 C. to 60xc2x0 C. or 35xc2x0 C. to 55xc2x0 C. The T(g) is determined by Differential Scanning Calorimetry (DSC) at a rate of heating equal to 18xc2x0 F. (10xc2x0 C.) per minute.
In another embodiment of the present invention, the polymer having reactive functional groups is a polyurethane polymer containing the functional groups mentioned above for the acrylic polymers. Polyurethane polymers can be prepared by reacting polyols and polyisocyanates. Examples of suitable polyols include low molecular weight aliphatic polyols such as ethylene glycol, propylene glycol, butylene glycol, 1,6-hexylene glycol, neopentyl glycol, cyclohexanedimethanol, trimethylolpropane and the like. Typically, high molecular weight polymeric polyols such as polyether polyols and polyester polyols are used with the lower molecular weight polyols. Examples of polyether polyols are those formed from the oxyalkylation of various polyols like glycols or higher polyols. Suitable glycols include ethylene glycol, 1,6-hexanediol, and Bisphenol A. Suitable higher polyols include trimethylol propane and pentaerythritol. Suitable polyester polyols can be prepared by the polyesterification of organic polycarboxylic acids or anhydrides thereof with organic polyols. Usually, the polycarboxylic acids and polyols are aliphatic or aromatic dibasic acids and diols.
Suitable polyisocyanates include aromatic and aliphatic polyisocyanates. Aliphatic polyisocyanates are preferred because of their exterior durability. Exemplary polyisocyanates include 1,6-hexamethylene diisocyanate, isophorone diisocyanate and 4,4xe2x80x2-methylene-bis-(cyclohexyl isocyanate).
Carboxylic acid functionality can be introduced into the polyurethane by reacting the polyurethane polyol with polycarboxylic acids. Exemplary polycarboxylic acids include succinic acid, adipic acid, azelaic acid, sebacic acid, terephthalic acid, isophthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, trimellitic acid and anhydrides of such acids. Alternatively, the polyisocyanate can be reacted with a mixture of the polyols mentioned above and a polyol containing carboxylic acid groups such as dimethylol propionic acid.
Hydroxyl functionality can be introduced into the polyurethane by reacting the polyisocyanate with a stoichiometric excess of the polyol component to form a polyurethane polyol.
Epoxy functionality can be incorporated into the polyurethane by including a hydroxy functional epoxy compound like glycidol with the polyol component.
Amino functionality can be introduced into the polyurethane by including a polyamine in the monomer charge. Suitable amines include primary and secondary diamines and polyamines in which the radicals attached to the nitrogen atoms are saturated, aliphatic, alicyclic, aromatic, aromatic-substituted aliphatic, aliphatic-substituted aromatic, or heterocyclic.
Pendant carbamate groups can be incorporated into the polyurethane by forming a hydroxyalkyl carbamate which can be reacted with the polyacids or polyols used to form the polyurethane.
Pendant urea groups can be introduced into the polyurethane by reacting a hydroxyl functional urea such as hydroxyalkyl ethylene urea with the polyacids and polyols used to form the polyurethane. Also, isocyanate terminated polyurethane can be reacted with primary amines, aminoalkyl ethylene urea, or hydroxyalkyl ethylene urea to yield a material with pendant urea groups.
The polyurethane polymers typically have number average molecular weights of about 3,000 to 25,000 or 5,000 to 10,000 based on gel permeation chromatography using a polystyrene standard. The polyurethane polymers have equivalent weights (based on the functional groups mentioned above) of about 280 to 2,805 or 1,122 to 1,870 gram/equivalent. The T(g) of the polymer is typically about 35xc2x0 C. to 85xc2x0 C. or 45xc2x0 C. to 60xc2x0 C.
In another embodiment of the invention, the polymer having reactive groups is a polyester polymer having the functional groups mentioned above. Polyester polymers are based on a condensation reaction of low molecular weight aliphatic polyols, including cycloaliphatic polyols, with aliphatic and/or aromatic polycarboxylic acids and anhydrides. Examples of suitable aliphatic polyols include 1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, cyclohexane dimethanol, trimethylol propane, and the like. Polymeric polyols such as the polyether polyols mentioned above can be used in combination with the low molecular weight polyols. Examples of suitable polycarboxylic acids and anhydrides include succinic acid, adipic acid, azelaic acid, sebacic acid, terephthalic acid, isophthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, trimellitic acid and anhydrides of such acids.
Carboxylic acid functionality can be introduced into the polyester by reacting a stoichiometric excess of the polycarboxylic acid with the polyol.
Hydroxyl functionality can be incorporated into the polyester by reacting a stoichiometric excess of the polyol component with the polycarboxylic acid.
Epoxy groups can be introduced into the polyester by including an epoxy functional compound such as glycidol with the polyol component.
Amino groups can be introduced into the polyester by including an amino alcohol such as amino ethanol or amino propanol with the polyol component.
Pendant carbamate groups can be introduced into the polyester by forming a hydroxyalkyl carbamate which can be reacted with the polyacids or polyols used to form the polyester.
Pendant urea groups can be introduced into the polyurethane by reacting a hydroxyl functional urea such as hydroxyalkyl ethylene urea with the polyacids and polyols used to form the polyester. Also, polyester prepolymers can be reacted with primary amines, aminoalkyl ethylene urea, or hydroxyalkyl ethylene urea to yield a material with pendant urea groups.
The polyester polymers typically have number average molecular weights of about 3,000 to 35,000 or 5,000 to 10,000 based on gel permeation chromatography using a polystyrene standard. The polyester polymers have equivalent weights (based on the functional groups mentioned above) of about 280 to 2,805 or 122 to 1,870 gram/equivalent. The T(g) of the polymer is typically about 25xc2x0 C. to 85xc2x0 C. or 50xc2x0 C., to 70xc2x0 C.
The powder coating composition of the present invention also comprises a curing agent having functional groups that are reactive with the functional groups of the polymer described above. Suitable curing agents include polyepoxides, beta-hydroxyalkylamides, polyacids, aminoplast, and blocked polyisocyanates.
Polyepoxides are suitable curing agents for polymers having carboxylic acid groups and amine. Examples of suitable polyepoxides include those described in U.S. Pat. No. 4,681,811 at column 5, lines 33 to 58, incorporated herein by reference.
Beta-hydroxyalkylamides are suitable curing agents for polymers having carboxylic acid groups. Examples of suitable beta-hydroxyalkylamides include those described in U.S. Pat. No. 4,801,680 at column 2, line 42 to column 3, line 9, incorporated herein by reference.
Polyacids, particularly polycarboxylic acids, are good curing agents for polymers having epoxy functional groups. Examples of suitable polycarboxylic acids include those described in U.S. Pat. No. 5,407,707 at column 3, line 55 to column 4, line 10, incorporated herein by reference.
Aminoplast and phenoplast curing agents are suitable curing agents for polymers having hydroxyl, carboxylic acid, carbamate and urea functional groups. Examples of suitable aminoplast include alkylated methylol melamine and alkylated methylol urea.
Blocked polyisocyanates and polyisocyanurate are suitable curing agents for polymers having hydroxyl and amino groups. Examples of suitable blocked polyisocyanates include benzene triisocyanate, polymethylene isocyanate, 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 4,4xe2x80x2-diphenylmethane diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, toluene diisocyanateo, 1,4-cyclohexyl diisocyanate, isophorone diisocyanate, and 4,4xe2x80x2-methylene-bis(cyclohexyl isocyanate).
The curing agent must be present in an amount sufficient to cure the powder coating composition of the present invention. Typically, the curing agent is present in the powder coating composition in an amount ranging from about 10 to 90 weight percent or from 25 to 75 weight percent, said weight percentages based on the total weight of resin solids in the powder coating composition.
The powder coating composition of the present invention also comprises a tricarbamoyl triazine compound represented by the following chemical formula: C3N3(NRCOXY)3 where Y is an alkyl group or substituted alkyl group having 1 to 12 carbon atoms, X is NRxe2x80x2, O, S, PRxe2x80x2, and R and Rxe2x80x2 are hydrogen and alkyl or a substituted alkyl having 1 to 12 carbon atoms. Examples of suitable substitution groups for the substituted alkyl include ether, ester, amide, acid, epoxy, urethane, urea, hydroxyl (preferred substituent), thiol, silyl (alkoxysilane) group, and cyano group.
When X is O, Y is preferably an alkyl group or a substituted alkyl group having 6 to 12, more preferably 6 to 8 carbon atoms.
When X is NRxe2x80x2, Y is preferably an alkyl group or a substituted alkyl group having 1 to 6 more preferably 2 to 4 carbon atoms.
Tricarbamoyl triazine compounds and methods of preparing them are described in U.S. Pat. No. 5,084,541, incorporated herein by reference.
It is significant that the present invention is limited to tricarbamoyl triazine compounds having the following chemical formula: C3N3(NRCOXY)3 where Y is ax alkyl group or substituted alkyl group having 1 to 12 carbon atoms, X is NRxe2x80x2, O, S, or PRxe2x80x2, and R and Rxe2x80x2 are hydrogen and alkyl or a substituted alkyl having 1 to 12 carbon atoms. Typically, tricarbamoyl triazine compounds are not compatible with powder systems. However, up to 10 percent of the disclosed tricarbamoyl triazine compound based on the total weight of resin solids in the powder coating composition can be loaded into the powder composition without crashing the system. The tricarbamoyl triazine compound is present in the powder coating composition in amounts of up to 10 percent by weight or from about 2 to 8 percent by weight or about 4 to 8 percent by weight, based on total weight of resin solids in the curable composition.
Optimally, the powder coating composition of the present invention can also include the following materials which are all well known in the art: pigments, fillers, light stabilizers, anti-oxidants, flow control agents, anti-popping agents, and catalyst.
The powder coating composition of the present invention is formed by melt blending the above-mentioned components. The melt blending process proceeds as follows. Initially, all of the components are blended in a high shear mixer such as a Henschel Blender. Then, the blended components are melt blended in an extruder at a temperature between 80xc2x0 C. and 130xc2x0 C. Next, the extrudate is cooled. Lastly, the cooled extrudate is pulverized into a particulate blend and ground to a particle size of 17 to 27 microns using a grinding mill such as an Air Classifying Mill ACM II available from Micron Powder Systems.
The powder coating composition of the present invention can be applied directly to a substrate such as wood, plastic, steel and aluminum. The finished powders can be electrostatically sprayed onto test panels and evaluated for coating properties.
The powder coating composition of the present invention exhibits improved mar resistance properties in comparison to conventional powder coatings.